1. Field of the Invention
The present invention relates to a method for manufacturing capacitors for semiconductor devices and, more particularly, to a method for manufacturing capacitors having a TaON dielectric film that exhibits both the improved electrical characteristics and the high capacitance values required for advanced semiconductor devices.
2. Description of the Related Art
As is well known, recent developments in semiconductor processing techniques have allowed the successful production of semiconductor products having increasingly high levels of integration. As a result, active research and development efforts continue to be directed toward both reducing cell area and reducing the device operating voltage. The reduction in cell area, however, has required a generally corresponding decrease in the area available for forming the capacitor while still requiring similar capacitance levels. In light of these competing demands, therefore, it has proven difficult to manufacture the more highly integrated memory devices.
Accordingly, there is a need for capacitors capable of providing sufficient capacitance in a reduced area to support the manufacture of highly integrated semiconductor devices, e.g. a memory device having more than 256M memory cells. In particular, the capacitance required for stable operation of the memory device is about 25 fF per cell. Capacitance values on this order are necessary to prevent the generation of soft errors and the corresponding reduction in refresh time, despite the decrease in the area available for forming the capacitor.
The basic capacitor structure, a dielectric layer interposed between a lower electrode and an upper electrode, is well known. Similarly, it is well known that the capacity of a given capacitor is proportional to both the surface area of the electrode and dielectric constant of the dielectric layer and is inversely proportional to the spacing between the electrodes, i.e. the thickness of the dielectric layer.
Efforts to increase capacity of the capacitor without increasing the cell size have included forming lower electrodes having complex three-dimensional structures such as cylinders, steps, and fins and the use of higher surface area materials such as hemispherical grain (HSG) polysilicon. Efforts to increase the capacitance have also included the use of dielectric materials having higher high dielectric constants.
However, because the process necessary to form capacitor lower electrodes having three-dimensional structures introduces additional difficulties and complications into the manufacturing process, recent efforts have focussed on developing a dielectric layer having a higher dielectric constant.
For example, a dielectric film having a multi-layer oxide/nitride/oxide (ONO) structure or a nitride/oxide (NO) structure are conventionally used as a dielectric layer of a capacitor. However, because the dielectric constant (E) of the NO structure is only 4xcx9c5 or so, it is difficult, if not impossible, to use an NO dielectric layer and still obtain sufficient capacity to prevent the generation of soft error and reduction in refresh time. Efforts are in progress to replace the NO dielectric layer and its 4xcx9c5 dielectric constant with a Ta2O5 thin film having a 25xcx9c27 dielectric constant. The higher dielectric constant of the Ta2O5 film permits the construction of capacitors having substantially higher capacitance than similarly sized NO capacitors.
The Ta2O5 thin film, however, deteriorates leakage current and dielectric characteristic of a capacitor. Consequently, it is difficult to use the Ta2O5 thin film as a dielectric layer.
In a nominal Ta2O5 thin film, however, substitutional Ta atoms inevitably exist as a result of composition ratio variations between Ta and O atoms within the film. The nominal stoichiometry simply does not reflect the chemical instability of the Ta2O5 thin film. In other words, substitutive Ta atoms in the form of oxygen vacancies are always locally present in the Ta2O5 thin film due to the variable and unstable stoichiometry of the Ta2O5 material. These oxygen vacancies then cause leakage current to be generated in a capacitor. Therefore, it is difficult to use the Ta2O5 thin film as the dielectric layer without additional treatment.
During the formation of the Ta2O5 thin film, the organic components from the Ta(OC2H5)5, a precursor compound commonly used in forming the Ta2O5 film, can react with O2 or N2O gas, so that impurities, that is, carbon (C), carbon compounds (such as CH4 and C2H4), and water vapor (H2O) are formed and incorporated into the thin film. As a result of these impurities, as well as other ions, free radicals, and the oxygen vacancies, in the Ta2O5 film, the resulting capacitors tend to exhibit increased leakage current and degraded dielectric characteristics.
For this reason, in order to prevent generation of leakage current in the Ta2O5 capacitors, it is necessary to perform both a separate oxidation process to stabilize the stoichiometry of the Ta2O5 thin film by oxidizing the substitutive Ta atoms present in the dielectric thin film and a series of low temperature thermal annealing processes. Using a Ta2O5 thin film as the dielectric layer, therefore, tends to complicate the whole process of manufacturing a capacitor. Further, an oxidation reaction may be generated at the interface between the lower electrode, usually polysilicon, and the Ta2O5 thin film during the low temperature thermal annealing process. This reaction, and the resulting formation of a thin oxide layer at the interface, further deteriorates the dielectric characteristics of the capacitor.
Consequently, a TaON thin film has been proposed as an alternative dielectric layer capable of obtaining the desired high capacity, low leakage current, and good dielectric characteristics. Like the Ta2O5 thin film, the TaON thin film is formed by the reaction between an organic metal compound (Ta(OC2H5)5) and O2 gas and, therefore, tends to include the same impurities such as carbon, carbon compounds and water vapor. Therefore, TAON thin films are typically annealed under N2O or O2 gas atmosphere to remove or reduce the incorporated impurities.
On the other hand, concentrations of atoms existing in the dielectric layer of a capacitor in accordance with the conventional art in reference to FIGS. 4A and 4B are described as follows.
As shown in FIGS. 4A and 4B, in comparison with compositions of atoms including Ta, O, N, C, Si in TaON thin film when compositing amorphous TAON thin film in the conventional art and carrying out furnace annealing of N2O, loss of nitrogen by furnace annealing is larger than that by depositing amorphous TaON thin film.
That is, a substitution Ta atom due to a difference in composition between Ta and O exists in TaON thin film.
Due to this, carbon atom and carbon compound including C, CH4O, C2H4 and etc, and H2O are generated due to reaction of organic of Ta(OC2H5)H5 in depositing a dielectric layer. Accordingly, a composition of nitrogen in TaON thin film is decreased, thereby increasing a leakage current of a capacitor and degrading largely a characteristic of a dielectric layer.
In the case of a TaON thin film, however, annealing under a N2O or O2 gas atmosphere can cause nitrogen to be replaced by activated oxygen, thereby lowering the effective dielectric constant of the film. The need to anneal the TaON film and the resulting degradation of the dielectric constant act to limit the capacitance levels that can be reached by a capacitor using such a TaON thin film.
Accordingly, it is the object of the present invention to provide a method of manufacturing a capacitor for a semiconductor device that exhibits improved leakage current and improved dielectric characteristics using a TaON thin film as a dielectric layer.
A method of manufacturing a capacitor of a semiconductor device according to the present invention to accomplish the aforementioned object comprises the steps of: providing a semiconductor substrate over which selected lower patterns are formed and an intermediate insulating layer is covered; forming a lower electrode on the intermediate insulating layer; depositing a TaON thin film in an amorphous state on the lower electrode; annealing the amorphous TaON thin film in a vacuum state to form a crystalline TaON thin film as a dielectric layer; and forming an upper electrode on the dielectric layer made of the TaON thin film.
The objects, aspects, advantages, and practice of the present invention can be understood more easily in light of the following detailed description and the attached figures.